Japanese Patent Application Laid-Open (JP-A) No. 2004-82332 describes a method for forming a foam molded article with a surface skin layer from a multilayered parison comprising a resin layer and a foamed layer, and also describes a method for integrally thermocompression bonding a fiber surface skin material with amass per unit area of 100 to 500 g/m2 integrally to a surface skin layer. JP-A-2001-97132 describes a package tray (rear parcel shelf) for vehicles, comprising a board in a foamed state molded with foaming injection molding. JP-A-2002-28997 describes a foam molded article with a surface skin formed by foaming injection molding. JP-A-10-29466 describes a method for forming a luggage box or a lid thereof for integral lamination of a surface skin material by charging foaming particles into a mold, and then heating with steam or the like to fuse the particles together.
In order to integrally attach a surface skin material to the surface of a foam molded article in blow molding, it is necessary to mold a thermoplastic resin currently maintained in a molten state at a temperature not less than a fixed temperature. However, when the temperature of the thermoplastic resin in molding is raised to a temperature sufficient for integral attachment of the surface skin material, the bubble cells will be broken, causing a rapid drop in a expansion ratio. Therefore, a blow molded article having a surface skin with lightweight and high rigidity cannot be obtained.
In addition, in order to realize a lighter weight, simple increase in the average particle diameter of the bubble cells and in expansion ratio deteriorates elongation property of the thermoplastic resin used. Since the resin is especially extended in a portion of the integrally formed reinforcing rib, there will be caused decrease in moldability, such as occurrence of a rupture, and simultaneously decrease in rigidity of the plate-shaped body as a base material, leading to occurrence of a large distortion to a load.